With the recognition that vitamin D is the precursor for the steroid hormone, calcitriol, it has become possible to characterize defects in the target actions of calcitriol. We have demonstrated a broad spectrum of manifestations of hereditary resistance to calcitriol. This syndrome usually results from a mutation in the gene for the vitamin D receptor. Skin fibroblasts from subjects with hereditary resistance to calcitriol all display abnormalities in this effector system, and defects in many discrete steps of this pathway have been identified with these cells. Cells with mutations in the calcitriol effector pathway can be used to explore mechanisms of calciferol action. They have been used to establish that the vitamin D receptor mediates an extremely rapid (within 1-3 min) rise of cyclic GMP in response to calcitriol and to demonstrate that cGMP plays a role in receptor translocation. These cells have also been used to show that certain receptor mutations compromise many receptor functions but allow another functions to be retained normally. This establishes that calcitriol receptors couple to different responses by distinct mechanisms. With the development of a new, pharmacologically relevant fluorescent calcitriol, it became possible to study vitamin D receptor activation in living cells and recognize abnormalities in vitamin D receptor translocation from the cytoplasm to the nucleus. The fluorescent calcitriol allowed the use of confocal microscopy to identify the association of vitamin D receptors with endoplasmic reticulum and microtubules, and to visualize intranuclear target sites.Transient expression of a green fluorescent protein glucocorticoid receptor chimeric protein allowed us to study the activation of another steroid receptor in real-time by microscopy, and allowed the generalization of our findings on vitamin D receptor activation process.